Ban Ki-moon has said the UN weapons inspectors investigating last week's suspected chemical weapons attack will leave Syria on Saturday, one day ahead of schedule.

The announcement deepened anticipation over imminent air strikes. Under an agreement with the Syrian government, the UN team had until the end of Sunday to complete their investigation.

If they leave a day early, they will not be able to carry out the three other site inspections from earlier suspected chemical attacks they had initially intended to complete.

The move is reminiscent of similar hasty departures of UN weapons inspectors from Iraq over a decade ago, after receiving a tip-off from western intelligence agencies that US air strikes against Saddam Hussein's regime were imminent.

Ban called on western powers to refrain from taking action until the UN inspectors had produced a report, but he added to the growing sense of urgency by saying the inspection team would report to him as soon as they left the country. UN officials, however, noted that a proper analysis of the chemical agents involved would take a lot longer. The inspectors had so far sent none of the samples they had collected out of the country for tests, but would be carrying the samples with them when they left. Laboratory work would only begin once they had left Syria.

It is possible however that Ban will ask for an interim report that could provide circumstantial evidence such as witness testimonies and the identification of the weapons and delivery systems used in the attack. That could provide part of what the UN has described as an "evidence-based narrative", from which member states could draw their own inferences. It is not clear whether the head of the inspection mission, Åke Sellström, would agree to such a partial report under intense pressure from the secretary general and some security council members.

Meanwhile, US and British government attempts to rally support for military action were damaged by the publication of an Associated Press report quoting unnamed US intelligence officials as describing the case against the Assad regime as no "slam dunk", adding that uncertainty remains over real control over Syrian chemical weapons stockpiles.

The basketball term "slam dunk", reflecting certainty of outcome, has haunting overtones in Washington as it was attributed to the then CIA director George Tenet in describing the intelligence case in the runup to the 2003 Iraq invasion for Saddam Hussein's supposed possession of weapons of mass destruction.

The report quoted "multiple US officials" as pointing towards gaps in the US intelligence picture, in strong contrast to the certainty expressed by American leaders, including Barack Obama, and said the US intelligence evidence against Syria was "thick with caveats".

"It builds a case that Assad's forces are most likely responsible while outlining gaps in the US intelligence picture," AP reported.

The developments come amid uncertainty over the threat of air strikes, which US and UK officials had suggested would happen within days, probably by the end of the weekend.

That timing has been put in doubt by a parliamentary revolt in London by both the Labour party and Conservative rebels.

Downing Street has suggested it has received US assurances that the White House is willing to wait until Tuesday next week to give the House of Commons time for a second debate on British involvement in air strikes, but Whitehall sources have said it is possible the US could go ahead without Britain, possibly with France, where the president, François Hollande – another proponent of punitive action – is not constrained by a parliamentary system.

Support for air strikes is mixed. The German chancellor, Angela Merkel, has urged a diplomatic solution, calling on Russia's president, Vladimir Putin, to compromise with the west to produce a consensual response to the crisis.

"The chancellor called on the Russian president to use negotiations in the UN security council for a quick, unanimous international reaction," a German government statement said.